In May of 1999, we were awarded an R21 grant to study and develop a prototype hardware configuration for low-cost magnetic resonance imaging called Prepolarized MRI. During the R21 support, we succeeded in building a Prepolarized MRI scanner for less than $30,000 and we succeeded in acquiring in vivo human hand images. The image quality is roughly comparable to a 0.2 T conventional MRI scanner and is rapidly improving. This R33 application is in response to the Development of Novel Technologies for In Vivo Imaging (PAR-07- 707) Phased Innovation Initiative. Here we propose to extend the technology development initiated in the R21 grant to develop an ultra-low-cost prepolarized MRI head scanner. Prepolarized MRI uses two pulsed magnetic fields. First a strong but inhomogeneous polarizing magnet is pulsed on to polarize hydrogen. This field is turned off and a very low field MRI acquisition follows. A prepolarized MRI scanner with a 0.5 T polarizing field should obtain the image quality of a conventional 0.5 T MRI scanner for a small fraction of the capital cost of a conventional MRI scanner. Here we propose to build a 0.5 T polarizing magnet and a low-field imaging MRI scanner for head imaging. We also plan to develop all the standard imaging pulse sequences that are currently used for detecting and following brain tumors. We plan to investigate the new contrast mechanisms that prepolarized MRI can offer. And finally, we plan to do a small study comparing image quality on a conventional 0.5 T GE MRI scanner. The results of this study will provide feedback for optimizing both the hardware and pulse sequences.